In gas turbine engines, such as aircraft engines, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel in a combustor. The mixture is then burned and the hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of combustion gas expands through the turbine, which in turn spins the shaft and provides power to the compressor. The hot exhaust gases are further expanded through nozzles at the back of the engine, generating powerful thrust, which drives the aircraft forward.
Because engines operate in a variety of conditions, foreign objects may sometimes undesirably enter the engine. More specifically, foreign objects, such as large birds, hailstones, sand and rain may be entrained in the inlet of the engine. As a result, these foreign objects may impact a fan blade and cause a portion of the impacted blade to be torn loose from the rotor, which is commonly known as fan blade out. The loose fan blade may then impact the interior of the fan casing at an impact zone, thereby causing a portion of the casing to bulge or deflect. This deformation of the casing may result in increased stresses along the entire circumference of the fan casing.
In recent years composite materials have become increasingly popular for use in a variety of aerospace applications because of their durability and relative light weight. Although composite materials can provide superior strength and weight properties, and can lessen the extent of damage to the fan casing during impacts such as blade outs, improvements can still be made.
Current containment technology, such as that used to manufacture fan casings, generally requires the use of a thicker casing design at high stress regions. More specifically, current fan casings are often made using a thick, monolithic hardwall design, which can help fragmentize a released fan blade and minimize the extent of damage. The energy generated by a released fan blade impacting a hardwall fan casing can be dissipated by any of several controlled failure mechanisms including resin microcracking, composite material ply delamination, and composite material ply failure.
All of the previously described energy dissipation mechanisms require the use of an untoughened resin to ensure controlled failure of the fan casing upon impact of the released fan blade. More particularly, an untoughened epoxy resin can be applied uniformly to the entire fan casing during resin application. After curing, the resulting composite material will have the previously described controlled failure mechanisms. However, areas away from impact zones, or non-impact zones, need to be stronger in order to maintain the integrity of the casing should an impact occur. Therefore, additional layers of untoughened composite material are often applied in non-impact zones in order to provide strength and toughness, as well as compensate for the lower strength of the untoughened material. These additional layers add undesired weight to the fan casing.
Accordingly, there remains a need for articles having increased strength in desired regions without increasing the overall weight of the article.